Physiology - Origin and Conduction fo Cardiac Impulse

Question 1

What is autorhythmicity?

Answer 1

The heart is able to generate electrical signals without an external stimuli. Slide 2

Question 2

What is the SA node and where is it situated?

Answer 2

The sino-atrial node. A specialised group of cells whcih act as the pacemaker. It is situated up at the top of the right atrium. Slide 6

Question 3

What happens in the SA node?

Answer 3

The spontaneous potential brings the membrane potential to the threshold which generates an action potential in the cells. Slide 10

Question 4

The permeability of the pacemaker cells for K+ always remains constant. True or False?

Answer 4

False, it does not remain constant between action potentials. Slide 12

Question 5

What causes the pacemaker potential?

Answer 5

Decrease in K+ efflux, increase in K+ and Na+ influx and a transient Ca2+ influx. Slide 12

Question 6

What happens when the threshold is reached?

Answer 6

The rising phase.

There is a Ca2+ influx resulting in depolarisation. Slide 13

Question 7

What happens during the falling phase of the action potential?

Answer 7

The long lasting C channels deactivate and the K+ channels open allowing an efflux of K+. Slide 14

Question 8

What is the pathway of the electrical current through the heart?

Answer 8

SA node
AV node
Bundle of His
Left and right branches of it
Purkinje Fibers. Slide 17

Question 9

How does the current spread from the SA node to the AV node?

Answer 9

Through Gap Junctions in the intercalated discs between cells. Slide 18

Question 10

What is the AV node and where is it?

Answer 10

Atrio-ventricular node which is a small bundle of specialised cardiac cells at the base of the right atrium.
It is the only way for the impulse to travel through the fibrous heart skeleton. Slide 19

Question 11

What does the AV node do and why?

Answer 11

It delays the conduction of the impulse so atrial systole precedes ventricular systole meaning blood can be pumped out efficiently. Slide 20

Question 12

How does the action potential of cardiac muscle cells differ from the pacemaker cells?

Answer 12

Remains at -90mV until exctied which causes the rising phase to +20mV due to Na+ influx not Ca2+. Slide 23.

Question 13

How many phases are there in ventricular muscle action potential?

Answer 13

5 phases:
Phase 0
Phase 1
Phase 2
Phase 3
Phase 4. Slide 24

Question 14

What is so special about Phase 2?

Answer 14

The plateau phase of the action potential.

Unique characteristic of contractile cardiac muscle cells and is the influx of Ca2+. Slide 25

Question 15

Which increases Heart Rate? Parasympathetic or sympathetic?

Answer 15

Sympathetic. Slide 28

Question 16

How do the vagus nerve and the heart interact?

Answer 16

The vagus nerve provides a continuous influence on the SA node when under resting conditions. Slows the heart rate down from ~100bpm to ~70bpm.
Also increases the AV nodal delay.
Slide 29+31

Question 17

If heart rate is less than 60 what does it mean? If it is higher than 100 what does it mean?

Answer 17

<60 = brachycardia
>100 = tachycardia. Slide 30

Question 18

What drugs can be used to speed up heart rate and how?

Answer 18

Increase = Atropine.
Decreases the slope of the pacemaker potential, elongating the gap between each action potential.
Slide 31+32

Question 19

What is the term for the effect of increasing frequency of action potentials?

Answer 19

Positive Chronotropic effect. Slide 34

Question 20

What does ECG stand for?

Answer 20

Electrocardiogram. Slide 37